This invention relates to a method of forming a continuous film from an aqueous admixture of an elastomeric latex and a phenolic resin.
It has long been recognized that glass fiber material makes an ideal reinforcement for rubber products such as automobile tires, power transmission belts and the like. In preparing glass fiber material for such applications, the individual glass fibers or groups of glass fibers in the form of strand, rope, cord, roving, fabric and the like are coated with a rubber adhesive to aid in bonding the glass to the elastomeric materials to be reinforced. The rubber adhesive generally comprises a resin and an elastomeric material to form a bond between the glass and the main body of material being reinforced.
Generally, in the production of fiber glass reinforcing cords or other bundle forms, individual fibers are coated with a sizing and then the fibers are brought together in bundle form. Commonly the sizing contains a coupling agent such as a silane, a lubricant or other ingredients to assist in the handling of the bundle during processing. The bundle is then coated by dipping or otherwise contacting it with a coating mixture containing an elastomeric latex and ahomogeneous resinous component.
The term "elastomer" as used herein is intended to mean and include both synthetic and natural rubber. "Natural rubber" as used herein is the elastic solid obtained from the sap or latex of the Hevea tree, the major constituent being the homopolymer of 2-methyl 1,3-butadiene (isoprene). "Synthetic rubber" as used herein is meant to encompass polymers based upon at least 2 percent of a conjugated unsaturated monomer. The conjugation being in the 1,3 position in the monomer chain and the final monomer in its uncured state having an extensibility of at least 200 percent and a memory of at least 90 percent when stretched within the extensibility limits and released instantaneously. The conjugated unsaturated monomers which are used in the preparation of synthetic rubbers are, but not limited to, chloroprene, butadiene, isoprene, cyclopentadiene, dicyclopentadiene and the like. Other olefins capable of free radical anionic or cationic interpolymerization into the polymer chain with the conjugated unsaturated monomer are useful in forming synthetic rubbers. These olefins are typically monoethylenically unsaturated monomers. "Monoethylenically unsaturated" as used herein is characterized by the monomer having one CH.sub.2 .dbd. C&lt; group. These monoethylenically unsaturated monomers are, but not limited to, the acrylic monomers such as methyacrylic acid, acrylic acid, acrylonitrile, methyacrylonitrile, methylacrylate, methylmethacrylate, ethylacrylate, ethylmethacrylate and the like. Monoolefinic hydrocarbons such as ethylene, butylene, propylene, styrene, alphamethylstyrene and the like and other functional mono unsaturated monomers such as vinyl pyridine, vinyl pyrrolidone and the like functional vinylic monomers.
Glass fibers are excellent reinforcing materials and are distinguishable from other fibrous reinforcing materials such as natural and synthetic organic fibers in that the glass fibers do not become elongated or deformed under stress to the extent that other fibers do. Unlike other fibers, particular combinations of glass fibers with encapsulating coatings cooperate to yield reinforcing materials that have greater tensile strength than either the glass or coating material alone. While other materials which are subject to substantial stress elongation are essentially limited in tensile strength to the basic strength of the bare fibers, even if coated, properly coated glass fibers have greater strength than the glass alone. For example, the low modulus of elasticity of glass may be exploited to provide reinforced tires having superior road performance if an appropriate coating medium is provided to transfer stresses to all fibers in the glass fiber cord so that loading throughout is substantially uniform. This phenomena is illustrated by the observation that a typical uncoated glass fiber cord G-75, 5/0, filament count 2,000, i.e. 2,000 filament strands of G fibers, (about 3.7 .times. 10.sup.-5 inches diameter), 7,500 yards per pound of glass! has a tensile strength of about 35 to 40 pounds by ASTM Test D578-52. This same cord when coated with an elastomer resorcinol-formaldehyde coating has a tensile strength of about 50 to 70 pounds.
A plurality of components are used in the coating composition for the glass fibers to impart various properties thereto. Among these components are elastomers, as previously described, and phenolic resins especially resorcinol-formaldehyde resins. Further, carboxylated polymers are sometimes added to the dip material to impart adhesion and improve tensile strength. Waxes are sometimes added to the dip formulation to provide stability to ultraviolet light. Due to the plurality of components used in coating compositions which determine the final properties of the cord, a great deal of formulation must be conducted in order to find an acceptable or improved coating composition for the glass fiber cord.
Because there are so many components and the extent of the interaction of the ratios and compositions of these components is not completely known, physical testing of the tire cord is necessary in order to determine if a product is acceptable for final use.
Testing methods of the cord are conducted both in the laboratory and in the field. Laboratory testing is composed of both the testing of the cord itself and the testing of the cord embedded in a rubber matrix. The normal testing of the cord itself is usually by a tensile strength measurement. When the cord is embedded in a rubber matrix the composite is tested for its flexibility in accordance with the Scott flex test. The Scott flex test involves taking strips of rubber cord composite and flexing this composite for the desired number of cycles. After the cycles have been completed, the composite is inspected for breakage of glass filaments. This test is used as an indication of how the glass fiber cord will perform in its final use, i.e., tires, or power transmission belts.
In testing the adhesion of the cord to a rubber matrix the cord is embedded in the rubber matrix and sectioned so that the interface of the cord and the rubber matrix can be pulled in opposite directions in an Instron.RTM. testing device. This test for adhesion is also considered to be indicative of how the cord will perform in its final application.
Although these laboratory tests have been found to be somewhat representative of the quality of the glass fiber cord, these testing methods are not always reliable in predicting the final performance of the glass fiber cord. Therefore, field testing of the cord in tires and in power transmission belts must be made.
In order to field test the cord, it must be used in the construction of a tire. Therefore, individual tires having the cords to be tested are fabricated and run through various destructive testing techniques such as riding a car with the tires to be tested over a course of cobblestones for a certain number of miles; running the tires to be tested at high speed and low speed and finally after the predetermined amount of driving time has been completed, the tires are X-rayed and inspected for broken filaments in the cord.
The laboratory testing techniques do not involve great expense in both time and materials; however, the field testing of the tires which is the true indication of the performance of the cord amounts to a great deal of cost due to the expense in building the tires and testing the tires. It has been hypothesized by workers in this area that if the physical properties of the glass fiber coating formulation could be determined, an indication of the final properties of the tires could be obtained. However, the continuous, uniform, free films of the coating composition have not heretofore been able to be produced. This is attributable to the fact that when the coating composition as such is coated on a substrate, in a film thickness acceptable for testing, on drying the film cracks and becomes discontinuous or forms a powder; therefore, losing all utility for any type of testing technique.
The instant invention provides a method of forming a uniform continuous, free film of adequate thickness to test the physical characteristics of the elastomeric latex - phenolic resin admixture in its cured state.